Casting defects frequently arise in complex castings as a result of shrinkage in the last regions of the casting to solidify. Ideally, in casting design, sections most distant from available liquid metal will solidify first, and liquid metal will be available to feed the solidifying metal to prevent shrinkage. This solidification process, in an ideal casting, continues until the heaviest and last-to-freeze section is reached.
Castings are not always ideal, and the heaviest section, if there is one, may not always be the last section to freeze. To deal with this, mold designers include risers, which provide reservoirs of molten metal to feed hot molten metal to the casting as it solidifies. The primary function of the riser is to feed molten metal to the casting as it solidifies. Thus, a properly designed riser should be the last portion of the mold system to freeze. Of course, riser metal does not contribute to metal yield, as it must be cut from the casting, even though it may be remelted.
Castings, including risers, are fed by gating systems that feed molten metal from the metal source, usually a furnace or pouring ladle into the mold. Molten metal enters the mold cavity through gating systems which usually are designed to avoid turbulent flow. Gating systems may be top gating systems, bottom gating systems, side gating systems and step gating systems. The latter permit molten metal to enter the mold cavity successively from bottom to top as the mold is filled.
Any mold system that reduces the size of risers or completely eliminates the use of risers while simultaneously providing sufficient molten metal to feed shrinkage during solidification may improve metal yield, making the process more efficient and cost-effective. Other advantages also may accrue from such a system.